What Is Peto's Paradox?

By Antonia Čirjak on May 10 2020 in Answer

Richard Peto worked at the University of Oxford when he developed the paradox.
Richard Peto worked at the University of Oxford when he developed the paradox.
  • The chance of cancer appearing in humans is much higher than it appearing in whales, or toher large mammals.
  • Richard Peto developed the paradox in 1977, while he was working on an overview of the multistage model of cancer.
  • Medical research confirms that the risk of getting cancer is in direct correlation to size of the body of the species members.

Peto’s paradox is named after the English statistician and epidemiologist Richard Peto, and it refers to the observation that the incidence of cancer isn’t tied to the number of cells in an organism at the species level. The incidence refers to the probability of cancer appearing in the organism. We can explain it by using an example.

The incidence of cancer, meaning the probability of cancer appearing, is much higher in humans than it is in whales. This holds true despite the fact that whales have many more cells in their bodies than humans. If the probability of cancer forming and appearing in someone were tied to the number of cells, it would be expected that those species with higher cell counts have higher incidences of cancer. 

The Paradox Of Size

Richard Peto worked at the University of Oxford when he developed the paradox. It was in 1977, while he was working on an overview of the multistage model of cancer. He noticed how humans were less susceptible to cancer than mice. This was strange because a human has many more cells than a mouse. Humans also live 30 times longer than mice, on average. When we expose two similar organisms to the risk of getting cancer, but one of them for a period that is 30 times longer, we should get the result that that organism has anywhere from a million or a billion times higher chance of getting cancer per every cell. 

However, this is not the case. Humans should have a much higher chance of getting cancer according to every logic, but it is not so. The probabilities of getting cancer in humans and mice are the same. This would seem that, in layman terms, humans have stem cells that are billion times more “cancer-proof” than those found in mice. This is implausible when viewed from a biological perspective, which creates this paradox. Peto concluded that the possible reasons for this are various evolutionary factors. 

Studies Confirmed The Paradox

According to multiple medical research, the risk of getting cancer is in correlation to body size within the members of the same species. There were multiple studies made throughout the 20th and 21st centuries that confirmed this. These studies were performed over more extended periods and using a large number of subjects, sometimes even in millions. They all concluded that cancer and height are correlated, which means that the number of cells influences the possibility of getting cancer. The same was found when studying certain breeds of dogs. Smaller dogs were less likely to get cancer.

However, when viewed across species, all rules are off. The relationship between size and cancer incidence disappears. Multiple studies confirmed this as well, with some showing small species of animals being more susceptible to cancer than those that are much larger. These studies basically confirmed everything Richard Peto was saying, thus making Peto’s paradox a reality. It is still not completely clear what causes this, but it is presumed that several factors are influencing it. The most prominent one comes from various evolutionary reasons that reduced the danger of cancer through multicellularity. Others take metabolism into account, but still, we have no conclusive evidence as to why Pet’s paradox takes place.

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